This invention relates to the pyrolytic deposition of silicon nitride onto a heated substrate and to substrates having uniform films of silicon nitride deposited thereon. More particularly, this invention relates to the deposition of silicon nitride by reacting a halosilane with ammonia in an evacuated system.
Silicon nitride (Si.sub.3 N.sub.4) is a dense, chemically inert, dielectric material of extreme hardness, low thermal conductivity and high resistance to molecular diffusion. These properties have made silicon nitride an attractive and valuable material for a wide range of applications. For example, it is useful in the fabrication of semiconductor devices as oxidation masks, capacitor dielectrics for bit storage, masking layers, polish retarders, etc.
Various methods for depositing silicon nitride are known, but while the processes described in the prior art are functional in certain applications, they present drawbacks in other areas. For example, it has been found to be extremely difficult to deposit silicon nitride onto semiconductor substrates in a manner that will allow a good growth rate, uniform deposition, and a high quality coating in an economical process.
Thus, the deposition of silicon nitride by the reaction of either silane or dichlorosilane with ammonia at a pressure of about 1 atmosphere is conventional. However, the aforementioned processes are not completely satisfactory for depositing silicon nitride on semiconductor substrates because of the high cost of the equipment, over-all processing costs including the necessity of employing a carrier gas, and low throughput. Furthermore, these processes result in poor thickness uniformity on individual wafers and from wafer-to-wafer.
Accordingly, E. Tanikawa et al in "Chemical Vapor Deposition In An Evacuated System", C.V.D. 4th International Conference, ECS, G. F. Wakefield and J. M. Blocher, ed., 261-273 (1973) describe the reaction of silane and ammonia in an evacuated system to deposit silicon nitride on silicon wafers. However, wafers treated according to this process have been found to have a thicker ring of silicon nitride around the edge of the wafer together with silicon or silicon nitride dust and boat marks on the wafers. Furthermore, for best results, wafer size and spacing in the furnace must be uniform in carrying out the deposition.
It has now been found in accordance with this invention that silicon nitride can be deposited to provide surprisingly unexpected results by employing a halosilane as a reactant and carrying out the process in a vacuum.